I don't know about any REQUEST METHOD in the HTTP PROTOCOL that could be used to list directories. More about the HTTP protocol here: http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol) You can download a file (GET) or upload a file (PUT) but there is not directory listing operation. A web server may generate you a directory listing "html page" that is not standard parsable data but even this directory listing is usually disabled by default. Once I needed a directory listing feature and I implemented it as a custom HTTP request (LISTDIR) with a python cgi script.

You would have to request the URL you're trying to get the directory list for, usually ending in a forward slash, like this:
http://www.yahoo.com/

IF folder browsing is turned on, you'll get back a page listing the directories in the root of the web site. You parse this text (it's just a string!) and pull out the directory names and links to subdirectories.

But, NOBODY in their right mind will turn folder browsing on for their site. Doing so just gives people who want to break into your site more information on how to do it.